The present invention relates generally to tubular, electrical-resistance, heating elements and is more particularly concerned with novel sheathed elements having superior performance characteristics, with a method of making these novel elements, and with a new magnesia-base composition having special utility as a thermally-conducting, electrically-insulating, packing material in these elements.
Heating elements of the type comprising an inner, electrically-resistive conductor, a surrounding layer of magnesia electrical insulation, and an outermost protective jacket are widely used in many industrial heating devices as well as in devices such as domestic ranges, dishwashers and water heaters. This type of heating element is much more durable than, for example, exposed resistance wire. Structurally, it usually includes: (1) a coiled resistance wire composed of alloys such as those made up of 20 percent chromium and 80 percent nickel; (2) compacted magnesia powder containing minor amounts of impurities surrounding the resistance coil as an insulator; and (3) an outer protective metal jacket.
Over the long period in which such elements have been in general use, they have been developed and improved to a state of good performance and service life, meeting high safety standards and competing with consistent success with gas and high-frequency current heating devices. At the same time, however, it has long been recognized that a substantial increase in the thermal conductivity of the magnesia insulation employed in these elements would be desirable. Increased thermal conductivity results in decreased wire temperature, a significant factor in length of service life of these elements. This objective, however, would have to be realized without incurring any substantial offsetting disadvantage of cost of production or operation, impairment of efficiency of these elements, or significantly reduced electrical resistance.